Closer than most people guess: Brazil (8,510,346 km²) is only about 10% larger than Australia (7,741,220 km²).
Australia sits in Oceania, Brazil in South America. On the old Mercator map the latitude gap distorted how large each felt beside the other; at true size the answer is simply 1.1×.
The old Mercator map treated both about equally (1.2× vs 1.04× inflation), so at least their relative sizes were preserved, even if both were drawn at the wrong absolute scale.
Australia fits 1× into Brazil.
Australia vs Brazil: key figures
| Measure | Australia | Brazil |
|---|
| True area | 7,741,220 km² · #6 | 8,510,346 km² · #5 |
| Mercator inflation | 1.2× | 1.04× |
| Apparent on Mercator | 9,544,924 km² | 8,810,761 km² |